Storage tanks must be inspected periodically to determine whether a tank is in need of replacement or repair. Inspections detect corrosion, fissures, cracks, and other anomalies in tank walls and floors. Inspection techniques may include acoustic, electrical and/or mechanical techniques. Inspection reports usually contain basic measurements and an estimate of metal loss.
Tanks must be empty for inspection because testing personnel must have access to the inside of the tank. Consequently, inspecting fluid filled tank walls becomes expensive and dangerous. In most cases, tank operations in at least two tanks must stop so that fluid from the tank to be inspected can be pumped into a holding tank. The tank to be inspected may have to be cleaned before inspection personnel can enter the tank. Inspection personnel will, more likely than not, be required to wear personal protection equipment and carry oxygen to inspect tanks. Once inspection is completed, fluid must be pumped from the holding tank into the inspected tank.
FIG. 1 shows a typical tank used by refineries, storage facilities, and pipelines. Other tanks include, but are not limited to, tanks on super tankers, off shore oil production platforms, Floating Production Storage and Offloading vessels (FPSOs), airport storage tanks and oil-transport rail cars. Volatile petroleum products that may be stored in these types of tanks include flammable and combustible liquids and gases, and may produce flammable or combustible liquids, gases, vapors or mists when mixed with air under normal atmospheric conditions. The normal operating temperature range for this type of system is −20 C to +60 C at normal atmospheric pressures of 980 to 1050 millibars. Taking a typical tank out of operation, cleaning, and inspecting it can cost in the neighborhood of $5,000,000.